1. Field of the Invention
This invention relates to chainsaws and, more particularly, to a chainsaw with a mechanism for selectively tensioning an endless cutting chain thereon.
2. Background Art
Chainsaws are commonly configured with a main housing that contains a power unit. The housing has operating handles thereon that allow a user to conveniently lift and reposition the chainsaw. A trigger is provided on one of the handles through which operating speeds for the chainsaw are controlled.
The housing includes a frame to which a chain bar is operatively connected. The chain bar has a track for guiding movement of a chain in an endless path. With the chain bar operatively positioned, the chain can be trained around both the bar and a toothed component that is driven by the power unit.
Virtually every chain bar is connected to its supporting frame so as to be translatable guidingly in a line generally parallel to the bar length. By selectively shifting the chain bar lengthwise relative to the frame in opposite directions, the endless path for the chain, defined cooperatively by the chain bar and toothed drive component, can be effectively lengthened and shortened. Shortening of this path allows the chain to be initially installed in a slackened state upon the bar. Thereafter, the bar is shifted an amount to produce a desired tension upon the chain, whereupon the chain is fixed, as by tightening one or more threaded components that positively bear the chain bar against the housing frame.
As the chainsaw is operated, there is a tendency of the chain bar to shift in a manner whereby the chain may become slackened. This condition may be the result of pressures applied to the chain bar during cutting operations that cause the chain bar to shift. Additionally, the chain has a link construction that is prone to becoming lengthened over extensive use periods.
To compensate for this condition, chainsaws are commonly made with mechanisms that facilitate loosening of the chain and re-tensioning after the initial installation thereof. Early versions of these tensioners were quite basic and required tools to loosen fasteners and retighten the same after the chain tension was reset.
Tool-operated tensioners, while functionally effective, have drawbacks. First of all, the user is required to keep on hand and available the required tool or tools demanded by a particular design to operate the tensioner. If the necessary tools are for some reason not available, potentially the chainsaw will be inoperable. Alternatively, a user risks that a slackened chain might become damaged or, in a worst case, separate from the bar as the chainsaw is being operated.
Further, even if the user has on hand the necessary tool(s), manipulation of a separate tool is inherently inconvenient. This is aggravated by the fact that heavy gloves are recommended when using chainsaws. Thus, the operator is normally required to remove his/her gloves preparatory to changing the tension on the chain. This is particularly inconvenient in cold weather conditions and/or when there is precipitation.
The inconveniences associated with handling separate tools prompted the industry to develop tensioners that are characterized as “toolless”. Toolless tensioning involves hand manipulation of components that at all times remain part of the chainsaw unit.
Those designing toolless tensioners are guided by a number of different criteria. First of all, the structure must be reliable and convenient to operate. A failure of a component of a toolless system may render the entire chainsaw unusable until a repair is effected. Further, the steps required to re-tension a chain must be such that they are less onerous than those to re-tension chains using separate tools.
It is also preferred that the tensioning system be relatively simple in terms of its design. Complicated mechanisms often become expensive by reason of the high cost of the parts themselves, as well as their assembly. Complicated designs inherently tend to be more prone to failure than those that are more simplified.
There are currently many different toolless tensioner designs available, including many which are patented and many in commercial form.
Common to many of these designs is the incorporation of a hand-graspable knob that can be turned to selectively release and fix the chain bar in a desired position. Various mechanisms have been developed to maintain the knob in a desired, tightened position. This has been accomplished through separate latching components, use of ratchet mechanisms, use of parts with engaging teeth, etc.
While the above types of mechanisms have been effective in terms of holding the operating knob in a desired position and preventing rattling thereof, they do have drawbacks. Use of separate latching components, aside from generally requiring a relatively complicated design, requires a user to releasably set the latch each time the chain is re-tensioned. This introduces a level of inconvenience to the operator.
Further, the cooperating structures on relatively movable components, that prevent loosening and rattle, generally require parts to act against each other in a manner whereby they are prone to wear, particularly after extended use of the chainsaw.
The industry continues to develop new designs for toolless tensioners. These new designs are motivated by a desire to offer an affordable toolless tensioning option that is reliable and convenient to operate. The design process is complicated by the fact that these objectives are often competing in nature.